Base stock for vitreous enamel coatings



tem. .1

BASE STOCK FOR VITREOUS ENAMEL COATINGS Allan E. Chester, Highland Park,111., assignor to Poor 8: Company, Chicago, 111., a corporation ofDelaware No Drawing. Original application June 14, 1951, Ser. No.231,646. Divided and this application Sept. 7, 1955, Ser. No. 533,030

2 Claims. (Cl. 29-194) This invention relates to vietreous enameling,more particularly to new and improved base stocks for vitreous enamelsproviding a new and improved method of controlling the adherence ofvitreous enamels to ferrous metals.

This application is a division of my application Serial No. 231,646filed June 14, 1951, now US. Patent No. 2,768,904, which application isa continuation-impart of my applications Serial No. 753,805, filed June10, 1947, now abandoned, and Serial No. 87,375, filed April 13, 1949,now US. Patent 2,639,264, the former being a continuation-in-part of myapplication Serial No. 689,524, filed August 9, 1946, now abandoned.

One of the principal problems in the art of vitreous enameling is tosecure proper adherence between the vitreous enamel coating and the basestock to which the enamel is applied without the use of a ground or gripcoat of vitreous enamel containing cobalt, manganese and usually nickel.It has been recognized that it would be desirable to employ as theenamel base stock ordinary mild carbon steels (also called low-alloy ormild alloy steels) containing the usual percentages of carbon, as found,for example, in SAE 1010 steels, SAE 1020 steels, SAE 1030 steels, coldrolled commercial quality sheets and cold rolled drawing quality sheets.(See A.I.S.I. Steel Products Manual, section 11, October 1948 issue,pages 34 and 36.) Thus, SAE 1010 steel, as defined by Metals Handbook,1948 edition, published by the American Society for Metals, page 307,has an average carbon content between 0.08% and 0.13% by weight; SAE1020 steel has an average carbon content between 0.18 and 0.23% byweight; SAE 1030 has an average carbon content from 0.28 to 0.34% byweight. The attempts which have been made heretofore to use such mildcarbon steels as a vitreous enamel base stock have met with littlesuccess due to warpage and distortion of the object being enameled.Warping and distortion have been accelerated, moreover, because of thenecessity of applying more than one coat of enamel in order to secureadherence.

Many efforts have been made both by enamelers and steel manufacturers toovercome the handicap of using a ground coat of vitreous enamel followedby a second or even a third coat of enamel. Special steels have beenspecifically produced and designed for enamel purposes. The mostsatisfactory vitreous enamel base stocks have been specially preparedfrom low carbon steels (containing say, 0.01% to 0.02% by weight ofcarbon) which are more expensive than many of the ordinary steels.

An object of this invention is to provide a new and improved vitreousenamel base stock.

A still further object of the invention is to produce a satisfactoryvitreous enamel article in white and pastel colors with but one coat ofenamel. Other objects will appear hereinafter.

In accomplishing these objects in accordance with this invention, it hasbeen found that improved results in the adherence of vitreous enamels toferrous metal base stocks, especially mild alloy steels, can be obtainedin a "atent v '9 relatively simple manner by: (1) surface etching theferrous metal base stock, preferably with a fine grained etch that mightbe described as a pin point etch and which in the case of a cold reducedsteel should preferably extend just below the surf-ace and not below therolling skin, the said etch being effected in the presence of anoxidizing compound of manganese, preferably a compound which is at leastpartially soluble in the etching bath; and (2) applying to the etchedsurface of said base stock, preferably after rinsing but before drying,a layer of a metal, or a compound of a metal reducible to the metal,which is reducing with respect to ferrous iron under the firingconditions (and hence is below ferrous iron in the electrochemicalseries) and does not flash off or volatilize rapidly at vitreous enamelfiring temperatures.

In the practice of the invention, especially good results have beenobtained by etching the ferrou metal base stock with an acid etchingsolution to which has been added an oxidizing manganese compound, e.g.,manganese nitrate, manganese sulfate and/ or manganese dioxide. Theetching should be carried out under conditions such that the etched basestock contains a substantially uniform pin point surface etch free fromsmudge and which does not have to be scrubbed.

In the second step of the process, excellent results have been obtainedby immersing the etched base stock from the first step in an aqueousbath containing dissolved nickel salts and a relatively high acidity,for example, a pH of 1.8 to 2, preferably at a temperature around 180F.l F. for a period of 5 to 15 minutes followed by rinsing, scrubbingwith water and drying prior to the firing of a vitreous enamel thereon.If the base stock is to be shipped or stored it may be oiled in whichcase, however, the oil should be removed prior to vitreous enameling.

The invention will be further illustrated but is not limited by thefollowing examples in which the quantities are stated in parts by weightunless otherwise indicated:

EXAMPLE I (a) Sheets of SAE 1020 and SAE 1010 cold reduced steel ofvarying gauges (18, 20, 24, and 26 gauge) were etched in a bath of 4%sulfuric acid by weight containing 29 cc. of 50% manganese nitratesolution (Mn(NO for each gallon of bath. The steel sheets were immersedin the bath at a temperature of 150 F., for periods within the range of310 minutes. Additional baths were prepared with varying quantities ofmanganese nitrate within the range from about 5 to about 50 cc. of 50%manganese nitrate for each gallon of bath. The treated sheets wereuniformly etched and free from smudge. Especially good results wereobtained in the range of 10-20 cc. of 50% manganese nitrate per 1300 cc.of 4% by weight of water of sulfuric acid. Five (5) cc. solution of 50%Mn(NO contains 0.75 gram of manganese and 1.73 grams of nitrate (N0while 50 cc. of the same solution contains 7.5 grams of manganese andabout 17.30 grams of nitrate. Since the 1300 cc. of 4% sulfuric acidcorresponds to about 1336 grams the percentage of manganese in the bathsemployed was within the range of about 0.05% to about 0.5% and thepercentage of nitrate in the bath was within the range of about 0.1% toabout 1.0% by weight of the bath.

([1) The steel sheets prepared as in (a) were rinsed with water andwithout drying were immersed in baths prepared by mixing the followingingredients in the proportions indicated:

and making up to one gallon of bath with water, followed Patented Dec.13, 1960 by the addition of 3.8 to 38 grams per gallon of ZnSO or CdSOwith or without the addition of 3.8 grams per gallon of lauryl sulfate,and adjusting to a pH of 1.8 to 2 by adding 96% sulfuric acid.

The baths were maintained at a temperature of. 180- 190 F; and the steelsheets (treated as in (a)) were immersed therein for periods within therange of 5 to minutes, after which they were removed from the-bath,rinsed and scrubbed with water and then dried.

(0) Steel sheets which have previously been treatedv as described in (a)and (b) were then sprayed with a suitable vitreous enamel composition,dried and fired.

As an example of a suitable vitreous enamelcomposition there may bementioned a molybdenum bearing.

The finishing and pouring temperature of this batch after smelting isapproximately 2100 F.

For coats applied by spraying the following mill batch may be used.

Frit parts by weight 100 Clay parts by frit weight 4 Sodium nitrite do0.5 Antimony opacifier do 4 Water d0 40 This is ground or milled to afineness of 0 to 25% residue on a 200 mesh screen, preferably 4 to 8%residue. The resultant composition is then applied at the rate of 15-30grams per square foot to the base stock prepared as previouslydescribed, dried and fired at a temperature of 1350-1500 F.

Good results are also obtained with an enamel containing approximately60% of a non-feldspathic glass and 40% of a feldspathic glass leadlesscast iron vitreous enamel.

A typical non-feldspathic frit is made from the following ingredients:

Parts by weight 510 10.9 ZnO 10.5 H 30 21.9 BaCO 35.0 Na SiF 22.6

A typical feldspathic frit is made from the following ingredients:

Parts by weight Feldspar 21.7 Borax 31.0 Quartz 7.0 Soda ash 2.4 Bariumcarbonate 8.6 Sodium nitrate 5.7 Zinc oxide 7.0 Fluorspar 7.6 Whiting4.5 Sodium zirconium silicate 4.5

The foregoing frits are made in the usual manner by mixing, smelting,hitting and drying the compositions.

The hits are then mixed together in the 60:40 ratio and the followingmill additions are made:

Frit parts by weight White clay parts by frit weight 7 Color oxide oropacifier "do-.." 0-20 Bentonite do A; Barium carbonate do 141, Water do43 173 cc. of 96% sulfuric acid 36.5 grams of MnSO .4H O, and 20.4 gramsof HNO made up to two gallons with water. The proportions of manganeseand nitrate present in this bath correspond approximately to 10 cc. of50% solution of Mn(NO per 1300 cc. of bath.

Excellent results were also obtained in the use of a bath of this typein combination with the other steps recited in (b) and (c) of Example 1.

EXAMPLE 111 The procedure was the same as in Example I except that thebaths in (a) were prepared by dissolving 22.3 grams of MnSO .4H O in 200cc. of water, and adding the resultant manganese sulfate solution to asolution of 10.6 grams of N-a CO in 100 cc. of Water. The resultantmixture was boiled to precipitate the manganese carbonate which wasfiltered, washed and dissolved in a solution of 12.6 grams or HNO in 50cc. of water.

The resultant solution was added to 4% H SO in an amount sufiicient togive two gallons of bath. The concentration of this bath corresponds toapproximately 6 cc. of 50% solution of Mn(NO in 1300 cc. of 4% H 80 Verygood adherence of the vitreous enamel to the steel was obtained whenbaths of this type were employed in conjunction with steps (b) and (c)of Example 1.

EXAMPLE IV Sheets of SAE 1010 and SAE 1020 cold reduced steel of varyinggauges (18, 20, 24 and 26 gauge) are etched in a bath composed of thefollowing ingredients:

35 grams ferrous sulfate (FeSO JH O) 6 grams zinc sulfatetZnsO jl-l O)0.8 gram manganese dioxide 9.2 grams sodium bisulfate (NaHSO l-I O) 35cc. commercial sulfuric acid 66 B.

1 gram isopropylnaphthalene sodium sulfonate (Santomerse No. 1)

made up to 2 gallons with water.

The steel sheets are immersed in the bath at temperatures of F. to 170F, preferably at F. to F.

The treatment in this first bath is preferably carried out for about 15minutes. Although the time of treatment may be vary over a wider range,it is preferably within the range of 12 to 25 minutes.

The pH of the surface etching bath is preferably about 5.0 to 5.5.

The treated sheets are withdrawn from the first bathand immersed in arinsing bath.

The metal sheets are taken from the second bath and,

without drying, are immersed in a third bath having the followingcomposition:

170 grams nickel sulfate (NiSO .6I-I O) 40 grams nickel chloride (NiCl.6H O) 17 grams boric acid (H 30 3.8 grams citric acid 7.0 grams zincsulfate (ZnSO JI-I O) 0.8 gram isoprc-pyl-naphthalene sodium sulfonate(Aerosol OS) made up to 2 gallons with Water.

The pH of this bath is maintained around 1.8 to 3.5, preferably around2.5 to 3.5. The steel sheets are immersed in this bath for periodswithin the range from 5 to 15 minutes.

The temperature of this bath is preferably within the range of 180 F. to200 F. They are then removed from the bath, rinsed, scrubbed with waterand dried, and are ready to be sprayed with suitable vitreous enamelcompositions, dried and fired. it is preferable to employ vitreousenamel compositions which can be fired at temperatures not higher thanabout 1500 F.

EMMPLE V Steel sheets (22 gauge) were treated on a large scale asdescribed in Example 1V using a lviesta pickler. The raw steel wasloaded and then immersed in the successive baths. Each bath consisted of4000 gallons of solution made up in the same relative proportions givenin Example 1V. After the treatment in the third bath, the steel wasunloaded, scrubbed, dried and stacked.

in order to compensate for losses of the chemical addition agents of thebath due to their adherence or reaction with the metal and drag-outlosses, additional quantities were added by increment to the first bathat the rate of 0.8 pound to 1.74 pounds per ton of steel produced of acomposition made by fusing together:

7.5 parts ZnSO .7H O 1.0 part MnO 11.5 parts IJaHSO .H O

In the third bath, additional zinc sulfate (ZnSO .7I-I O) was added inincrements at the rate of .44 to .51 pound per ton of steel produced.The citric acid content was maintained at about 1 gram per liter. Nickelchloride (NiCl .6H O) was added at the rate of .17 to .51 pound per tonof steel produced. Nickel sulfate (NiSO .6l-1 O) was added at the rateof 8.3 to 9.9 pounds per ton of steel produced. Boric acid was added atthe rate of .11 pound to .4 pound per ton of steel produced. Aerosol OSwas added at the rate of .033 to .042 pound per ton of steel produced.

The ferrous sulfate will tend to build up in the first and third baths.In the first bath, a small amount of ferrous sulfate is desirable butmore than 2 pounds per gallon interferes with the activity of the bath.If the amount of ferrous iron becomes too high in the third bath, aspongy deposit of iron is formed on the sheet which interferes with thebonding power of the subsequently applied vitreous enamel. The citricacid permits the operation of this bath over a longer period of time bypreventing the precipitation of ferrous sulfate. Where no citric acidwas used, the practical limit of ferrous iron concentration was gramsper liter, while with the employment of citric acid, this limit may beincreased to 30 grams per liter or even higher in some cases to 45 gramsper liter, but care must be exercised to insure that the concentrationis below the point' at which a spongy deposit of iron occurs.

The surface tension :of the third bath is preferably maintained at 30drops per minute or greater as measured on a Stalagometer.

The pH of this bath may be around 2.0 to 2.5 but may vary over a widerrange of 1.8 to 3.5. At a pH around 6 4.0 there is a very sharp drop innickel deposition. At a pH of 4.5 to 5.5, however, excellent results areobtained with proper buffers.

The quantity of citric acid employed should not be any more than isemployed to keep the iron in solution or to prevent precipitation of theferrous sulfate in the above mentioned operating range. One gram ofcitric acid per liter of solution will usually be sufficient. Too muchcitric acid interferes with nickel deposition where the nickel is beingdeposited by over-voltage as in the present case. Formic acid and othersubstances which have a sequestering or an inhibiting effect on theprecipitation of ferrous sulfate may be employed instead of the citricacid.

By a procedure of the type described in this example, the steel is givena sunface etch and manganese salts are deposited thereon and sealed witha nickel plate of approximately 0.01174 to 0.01267 ounce per squarefoot. The thickness of this nickel plate in terms of millionths of aninch is of the order of 8.0 to 8.6. The sheet containing the nickelcoating has a bright metallic finish. The nickel coating may be removedreadily with nitric acid solution and after this has been stripped, thesheet exhibits a dark grain etch.

EXAMPLE VI This example illustrates the process of the invention Where anickel-cobalt alloy is plated by over-voltage in the presence of acathode depolarizcr.

Ferrous metal base sheets similar to those used in Examples IV and V,but in the form of test plates 1 /2" by 4 sq. ft.) were immersed insuccessive baths in three stages.

The first bath had the same composition as the first bath described inExample 1V except that it contained 205 cc. of 60 Baume' commercialsulfuric acid. Fifty milligrams of ferrous sulfide were added. Theferrous sulfide decomposes, libenates hydrogen sulfide which assists inactivating the sheet. Instead of ferrous sulfide, sodium polysulfide maybe used. The temperature of the bath was maintained at to F. The sheetswere treated in 15 to 20 minute cycles, that is, they were immersed forthat length of time.

After the treatment in the first bath each sheet was rinsed and immersed(in water containing 0.3% H SO The surface of the sheet may be regardedas containing myriads of couples with the subsequent deposition in thethird bath occurring only on those areas of the proper polarity. Thesame time period was employed in the second bath, that is, 15 to 20minutes.

The third bath consisted of the following ingredients:

37.5 grams CoSO .6H O

20 grams boric acid 3.8 grams citric acid 3.75 grams dextrose 25 cc. ofa product made by dissolving 40 pounds of sodium hydroxide in 8 /2gallons of water, adding chromic acid (CrO and then mixing with 37gallons of 50% gluconic acid. (See my US. Patent 2,428,356.)

50 mgs. sodium bisulfiite (NaHSO made up to two gallons with water.

The pH of this bath was maintained at 4.9 to 5.1 and the treatment againwas in a 15 to 20 minute cycle. In this bath the use of the dextrose isoptional. The compound derived by the reaction of the sodium hydroxide,chromic acid and gluconic acid acts as a depolarizer. The sodium'bisulfite functions as a reducing agent. This bath deposits alloys ofnickel and cobalt by over-voltage onto the ferrous base stock previouslytreated in the first and second baths. For the purpose of thisinvention, excellent results have been obtained where the alloy platedconsisted essentially of 90% nickel and 10% cobalt or 92.8% nickel and7.2% cobalt.

In the foregoing examples, cold rolled commercial quality and drawingquality sheets may be used with .excellent results. One such steel isspecified as .08 carbon max. and .32 to .42 manganese, as described inA.I.S.l. Steel Products Manual, sec. 11, October 1948, pp. 34 and 36.

The term mild alloy steels as used herein is intended to cover steelshaving an average carbon content in excess of 0.02% and not more than0.45%, preferably 0.07 to 0.20%, including the steels having thefollowing specific compositions:

COIIZPOSZIZOHS O Mn Si P Ni Cr Cu M .09 Low .55 .095 .45 0. l0 0. 25 0.75 0. l 0 75 0. 40 0.12 0.60 .08 0.40 0. 05 0. 35 0.95 I" 80 0.40 0. 0.70 0. 17 C 50 0. 00 0.10 0.60 0.15 0 l2 1.10 0.14 0.80 0.15 0. 12 0.300.10 0.60 a 0.10 0 60 0.60 0. 10 0. 35 0. 0S 0. 35 0 l2 0. 0.13 0.900.15 i "h i... 0.20 0.10 0. '55 0. "i 10 0. 50 75 0. 50 0. 13 0. 70 0.30 a. 0. 20 0. 60 0. 20 0.10 1.10 0.05 J12 i 0.3-5 0.12 0.75 0 75 1. 400.10 0.15 1.00 0. 50 0.25 0.10 0.08 0. 50 0.25 2.0 1.00 0. 07 0. Trace0. 06 0. 75 0.

The metal which is to be treated may be a regular cold rolled boxannealed steel sheet of the type commonly referred to as autobodysheets. The control of the composition of the metal, the annealing ofthe sheet and the temper rolling, if any, will be determined from therequirements of the end use of the sheet, especially as to the amount ofdrawing and firing which has to be done. Temper rolling of the sheet maybe per-formed before'or after the processing herein described, whicheveris more convenient in a particular plant.

Although the invention is not limited to any theory, it is believed thatthe minute cavities formed by the etching or pickling entrain a part ofthe pickle solution containing manganese nitrate or other manganesecompound. The aforesaid entrained compounds are then sealed by thecoating resulting from the deposition of a film of nickel or other metalbelow iron in the electrochemical series by the second step of theprocess. The entrained particles and the sealing layer are thereforeboth available to assist in controlling the oxidation and reductionreactions that occur when the vitreous enamel is fired on the ferrousmetal sheet thereby increasing the adherence and making it possible toprepare a commercially usable sheet of steel covered with a single coatof vitreous enamel.

In the step of the process whereby there is applied a thin layer orcoating of a metal which is reducing with respect to ferrous iron, othertypes of baths can be used to accomplish a similar purpose.

Among the preferred reducing metals which may be employed in thepractice of the invention are antimony, arsenic, cobalt, nickel or analloy of two or more of these metals. The thickness of the coating ofreducing metal is usually around 0.000001 to 0.000010 inch although itmay vary somewhat. The weight of metal per square foot will also varydepending upon the weight of the particular reducing metal used but inthe case of nickel or nickelcobalt alloys will'usually be within therange of 21 to 210 milligrams per square foot.

The plating step of the process which has been illustrated by theexamples as involving merely the immersion of the treated metal sheet ina bath represents the simplest way of carrying out this step of'theprocess.- It will be understood, however, that the coating of reducingmetal can be applied by other methods, such as, for example, byelectroplating, or by vaporizing the metal onto the sur face of thepreviously treated sheet where the natureof the metal lends itself tosuch a process.

The specific types of enamels given in Example 1(0) are illustrative ofmolybdic oxide-containing enamels and high fluorine enamels which havegiven especially good results in the practice of the invention but itwill be understood that the invention in its broader aspects is notlimited to these specific types of enamels.

It will be understood that in the examples some of the ingredientsrecited are optional. Thus, zinc sulfate or cadmium sulfate are employedin the plating step of the process to promote a smaller grain of thecoating metal. The sulfonated lauryl alcohol is employed as a surfacetension reducing agent to permit more penetration into the intersticesof the etched surface. Boric acid increases the efficiency of the bath.Other substances, which are 'known in the art of electroplating as beingefiective in reducing grain growth of electrodeposited metals can beemployed in the immersion baths of the present invention and contributea marked improvement in the result. The amount of metal deposited froman immersion bath, however, is not such that these substances functionin the same Way, viz., as brighteners.

It will also be understood that the ingredients and concentrations ofthe various baths employed in the different steps of the processesherein described may be varied within the bounds of the inventionpreviously set forth. In general, it is desirable that the etching orpickling bath employed in the first step of the process contain aslittle sulfuric acid as is required to accomplish the result and in anycase the quantity of sulfuric acid present should be such as to avoidpickling blisters.

The use of a metal which does not deposit on the ferrous iron but servesas a driver electromotive force to cause ion interchange between theiron and other metals is desirable in the practice of the invention. Forexample, in the surface etching bath where the manganese has been addedas manganese dioxide, it actually exists in solution as manganesesulfate. The ferrous ion is oxidized to the ferric ion. Zinc in the formof its sulfate in this bath has a high over potential and acts as adriver electromotive force for ion interchange between the iron and themanganese. Again in the plating bath where, for example, nickel is to beplated, the zinc acts as a catalyst because it has a higher overvoltagethan nickel in relation to iron. In a similar manner, cadmium, aluminumand/or magnesium in the form of their soluble salts, may be employed inplace of zinc. The quantities used are preferably 0.1 to 1 gram perliter of zinc sulfate (ZnSO .7H O) or the chemically equivalent amountof the other metal salts mentioned.

The importance of the invention resides in the fact that it provides avery simple and economical method of increasing the adherence ofvitreous enamel coatings to ferrous metal base stocks without the use ofground or grip coats of vitreous enamels containing cobalt, manganeseand/or nickel. As a result, it is possible to prepare vitreous enamelcoated articles containing only a single coat of vitreous enamelpossessing excellent adherence to the base stock. The simplicity of theprocedures involved in securing this adherence is such that the enameleris not required to depend upon the use of a special type of base stock.The base stock used can be similar to that now employed in makingautomobile bodies. Another important advantage of the invention is thatit can be applied in the treatment of welded shapes.

The invention is hereby claimed as follows:

1. A base stock for vitreous enamel coatings comprising a cold reducedmild alloy steel having an average carbon content in the range of0.07-0.20%, the surface of said steel being acid-etched to a finegrained pin point etch extending below the surface but not below therolling skin, said etched surface containing an oxidizing compound ofmanganese selected from the group consisting of manganese nitrate,manganese sulfate, and manganese dioxide deposited on said acid etchedsurface from the acid-etching bath at a concentration in said bathequivalent to ODS-0.5% manganese by weight of a thin coating over saidetched surface containing the deposited oxidizing compound of manganeseof at least one metal below ferrous iron in the electrochemical seriesand selected from the group consisting of antimony, arsenic, cobalt andnickel.

2. A base stock on which vitreous enamel coatings can be fired directlywhich comprises an article of mild alloy steel having an average carboncontent in the range of 0.02-0.45%, said article having a fine grainedpin point acid etched surface containing an oxidizing compound ofmanganese selected from the group consisting of manganese nitrate,manganese sulfate, and manganese dioxide deposited on said acid-etchedsurface from the acid-etching bath at a concentration in said 10 bathequivalent to ODS-0.5% manganese by weight, and a thin coating over saidetched surface containing said oxidizing compound of manganese of atleast one metal below ferrous iron in the electromotive series andselected from the group consisting of antimony, arsenic, cobalt andnickel.

References Cited in the file of this patent UNITED STATES PATENTS2,032,256 Canfield Feb. 25, 1936 2,101,950 McGowan Dec. 14, 19372,206,597 Canfield July 2, 1940 2,279,935 Belding Apr. 14, 19422,495,762 Porter Jan. 31, 1950 2,495,835 Comstock Jan. 31, 19502,495,837 Porter Ian. 31, 1950 2,639,264 Chester May 19, 1953 2,775,531Montgomery Dec. 25, 1956

1. A BASE STOCK FOR VITREOUS ENAMEL COATINGS COMPRISING A COLD REDUCEDMILD ALLOY STEEL HAVING AN AVERAGE CARBON CONTENT IN THE RANGE OF0.07-0.20%, THE SURFACE OF SAID STEEL BEING ACID-ETCHED TO A FINEGRAINED PIN POINT EACH EXTENDING BELOW THE SURFACE BUT NOT BELOW THEROLLING SKIN, SAID ETCHED SURFACE CONTAINING AN OXIDIZING COMPOUND OFMANGANESE SELECTED FROM THE GROUP CONSISTING OF MANGANESE NITRATE,MANGANESE SULFATE, AND MANGANESE DIOXIDE DEPOSITED ON SAID ACIDETCHEDSURFACE FROM THE ACID-ETCHING BATH AT A CONCENTRATION IN SAID BATHEQUIVALENT TO 0.05-0.5% MANGANESE BY WIEGHT OF A THIN COATING OVER SAIDETCHED SURFACE CONTAINING THE DEPOSITED OXIDIZING COMPOUND OF MANGANESEOF AT LEAST ONE METAL BELOW FERROUS IRON IN THE ELECTROCHEMICAL SERIESAND SELECTED FROM THE GROUP CONSISTING OF ANTIMONY, ARSENIC, COBALT ANDNICKEL.